1. Field of the Invention
The present invention relates to an ink jet recording apparatus and more particularly to a recording apparatus for recording by whole or part of ink drops ejected from the recording head are controlled or collected by means of electrostatic power, and an ink drop control method and an ink mist adsorption method, both for use in the apparatus.
2. Description of Related Art
Conventionally, an ink jet recording apparatus is known as to record characters and images by ejecting ink fluids in the recording head toward the recording medium. In comparison with another recording apparatus, this apparatus has such advantages as enabling (1) high speed recording, (2) multiple-color recording, (3) recording on ordinary papers that has not been specially processed, and (4) noise-free recording operations.
Such an ink jet recording apparatus has at least a recording head having an orifice from which ink fluids are ejected. The recording head ejects ink drops from the orifice selectively in responsive to recording information supplied to the recording head itself in order to form characters and images onto the recording medium.
An ink jet recording apparatus using electrostatic force for ejecting ink fluids is disclosed in Japanese Patent Application Publication No. 13768/1961.
An ink jet recording apparatus which enables the increase of the recording quality by accelerating the ejected ink drop by electrostatic force developed by a designated electric field applied to the ink drop ejected by the electro-thermal conversion device is disclosed in Japanese Patent Application Laying-Open No. 46257/1985.
A structure for the recording method using the electro-thermal conversion device and electrostatic force is disclosed in Japanese Patent Application Laying-Open No. 151348/1987.
A method for ejecting ink fluids by electrostatic force developed by the electric field generated at the time when the ink fluids are heated up to the high temperature by the electro-thermal conversion device is shown in Japanese Patent Application Laying-Open No. 225353/1987.
Some recording methods involve using charging electrodes for transferring charges to the traveling ink drops and deflecting electrodes for deflecting charged ink drops.
In a prior art recording method not using electrostatic forces, a single ink drop ejected from the orifice of the recording head in a single ejection operation may be broken into several pieces of ink drops during its traveling. Among these broken pieces of ink drops, fine-sized ink drops located in the backward side of the ink drop mist tend not to travel in straight lines but to reach positions on the recording medium different from positions to which large-sized ink drops are projected, which leads to a reduction in the recording quality of recorded characters and images.
In a recording method where ink fluids are extracted for ejection by electrostatic forces and/or ejected ink fluids are forced by electrostatic forces, the electric field developed for ejecting the ink fluid or accelerating the ink drop between the orifice of the recording head and the recording medium may disturb the trajectory of traveling ink drops. Specifically,
(1) the amount of ink fluids transported from the orifice of the recording head to the recording medium is less than that in the case where the electric field does not exist; or PA1 (2) for satellite drops, the backward part of the ejected ink drop goes back to the orifice of the recording head and blocks the ordinary ejection operation of ink fluids. More specifically, due to ink fluids piled up on and near the orifice, the trajectory of ejected ink drops may not be straight or, in the worst case, the ink fluids can not ejected at all. PA1 a voltage control means which applies a first voltage during a time period from a time when an ink fluid is ejected from the orifice to a time when the ink fluid is separated in 2 or more ink drops during its traveling, and immediately after applying the first voltage, applies a second voltage which has an identical polarity to a polarity of the first voltage and has an absolute value less than an absolute value of the first voltage.
FIG. 1 shows an outline of the prior-art ink jet recording apparatus using electro-static forces.
In FIG. 1, a recording head 1 has a plurality of nozzles 10 which are arrayed perpendicularly to a front surface of the recording head 1. In this case, the recording head 1 has four nozzles 10. Each of the nozzles 10 has an orifice 21 which is formed at the front thereof. A resistor 2 is disposed within the nozzle 10 and serves as an electro-thermal conversion device. Each of the resistors 2 is connected through a wiring 11 to a resistor driving circuit 3 for driving the resistor 2. A common fluid chamber 22 is disposed within the recording head 1 and is connected to all of the nozzles 10. The common fluid chamber 22 is connected to an ink supply tube 23 for introducing an ink 24 from a tank (not shown) into the common fluid chamber 22. A first electrode 6 is provided in the common fluid chamber 22. A second electrode 8 is disposed at a distance of d from the recording head 1 and is opposite to the front surface thereof as shown in FIG. 1. A recording medium 7 is placed on a surface of the second electrode 8. The first and second electrodes 6 and 8 are electrically connected to each other. By applying a voltage to the resistor 2, an ink drop 4 is ejected. A voltage V is applied between the first and second electrode 6 and 8, and the ejected ink drop 4 is accelerated to reach to a surface of the recording medium 7. The faster the ink drop travels, the higher the recording quality of recorded characters and images are.
FIG. 2 is the side view of what is shown in FIG. 1. FIG. 3 is a diagrammatic view of the ink drop ejection state of a specific nozzle in case that the voltage V is 0. What is shown in FIG. 3 is the trajectory of the ink drop at the time between 10 and 100 .mu.sec after the signal for requesting the ejection of the ink drop is received by the recording head. The distance, d, between the top of the orifice, x=0, and the surface of the recording medium is 0.5 mm. At t=50 .mu.sec, the ejected ink drop is completely separated into the main drop at the forward part and the satellite drop in the backward part, and both separated drops reach the recording medium.
However, due to the air flow generated by the relative movement of the recording head in the recording operation to the recording medium 7, the satellite drop may be projected onto unexpected positions in a definite probability or may go back to the top face of the nozzle of the recording head 1.
In contrast to FIG. 3, what is shown in FIG. 4 is the case where the voltage V is+1000 V in order to form an electric field E with its intensity being 2000 V/mm(=1000 V/0.5 mm). In case shown by FIG. 4, the ink drop is separated at the time t=50 .mu.sec. After 50 .mu.sec, the main drop is accelerated until reaching the recording medium. The speed of the satellite drop is reduced and finally moved backward to the orifice 21 until reaching to the orifice 21 of the recording head 1. As a result, the phenomena (1) and (2) described above arise. These phenomena are considered to be caused by the mechanism such that an extended ink fluid to be separated in drops is polarized dielectrically, and immediately after that, two distinctive parts, the major drop and the satellite drop, are developed, where the major drop is charged with the opposite polarity to that of the surface of the recording medium and the satellite drop is charged with the identical polarity to that of the surface of the recording medium.